Although type 2 diabetes mellitus (T2D) was formerly a disease exclusively of adulthood, youth-onset T2D now represents a substantial percentage of new cases of diabetes in children. When T2D onset occurs in youth (1) there is a substantial risk that diabetic complications will lead to major morbidity, mortality, economic loss, and reduced quality of life by young adulthood, (2) the consequences may disproportionately affect ethnic minorities, and (3) ?-cell failure and development of diabetes-related complications and comorbidities may be more rapid than in either adult-onset T2D or youth-onset T1D. Existing studies have been inadequate to draw definitive conclusions regarding rates of comorbidities, their long-term impact, and hard clinical endpoints. The TODAY clinical trial recruited 699 youth to address treatment modalities and start to document disease progression. Long-term observational follow-up (TODAY2) of this large, thoroughly characterized, cohort closely followed from near disease onset is an unparalleled opportunity to test the hypothesis that youth-onset T2D represents a more aggressive presentation of the disorder by direct comparison to equivalent longitudinal studies in adult-onset T2D and youth-onset T1D. In addition, the extensive phenotyping and close monitoring of this cohort from soon after diagnosis will address the question of whether the apparent difference in youth- onset T2D represents biological difference, is a consequence of poor metabolic control, or is related to the pubertal state. Longitudinal follow-up will also provide the opportunity to more fully understand the effects of puberty and transition out of puberty on youth-onset T2D, as well as the disease course of individuals who appear to have maintained durable control into the post-pubertal years. Finally, study of this cohort will add to our understanding of the challenges and burdens borne by individuals with youth-onset T2D as they transition into adulthood. To address these outcome questions, TODAY2 will follow the TODAY participants at annual in- person visits, semi-annual telephone contact, and quarterly telephone and electronic communications for a comprehensive and systematic approach to documenting outcomes indicating the progression of T2D, ?-cell function, and diabetes-related comorbidities and complications.